Below is a collection of available royalty-free images and cover art:

From Wizards of the Coast, zip files of background images:

For the Eberron campaign world: https://media.wizards.com/2020/dnd/downloads/dnd_backgrounds1_eberron.zip

For the Avernus campaign: https://media.wizards.com/2020/dnd/downloads/dnd_backgrounds_dia.zip

For Waterdeep: https://media.wizards.com/2020/dnd/downloads/dnd_backgrounds1_waterdeep.zip

From the core rulebooks (single jpg files) https://media.wizards.com/2020/dnd/downloads/DnD_bg-PHB01.jpg https://media.wizards.com/2020/dnd/downloads/DnD_bg-DMG01.jpg

Backdrops of the sets from various BBC shows including Doctor Who

https://www.bbc.co.uk/archive/empty_sets_collection/zfvy382

From Studio Ghibli:

http://www.ghibli.jp/info/013251/

From Chaosium (see bottom of the page):

https://www.chaosium.com/online-gaming-getting-started/

Star Wars:

https://www.starwars.com/news/star-wars-backgrounds

And the Nerdist did a nice digest of links to various backgrounds like Star Trek, Disney and more here:

https://nerdist.com/article/zoom-conference-backgrounds-star-trek-simpsons-seinfeld/